As used herein, the term “spiral bevel gears” is to be construed to include both non-axially-offset and axially-offset bevel gears. If one wishes to differentiate between the two types, the axially-offset bevel gears are referred to as hypoid gears.
Generally, there are two production processes for spiral bevel gears. In one case, tooth spaces of a pinion and ring gear of a bevel gear pair are each produced in generating processes, in the other case, the tooth spaces of the ring gear are only produced by plunging the rotating cutter head into the stationary workpiece, while in contrast the pinion gaps are produced in a special generating process using a tilted cutter head. In both cases, the generating process is based on an imaginary producer gear, which executes a generating movement with the workpiece during the processing in the gear cutting machine. In the first case, the producer gear is a flat toothed disk, in the second case, it corresponds to the mating gear, the ring gear produced in the plunging process.
This second production process was mainly developed for the automobile industry in order to save processing time. In comparison to a plunging process, a generating process takes significantly longer, production time adds up over the many teeth that must be produced for a ring gear. One reason for the longer generating time is the lower processing volume per tool rotation, another reason is the longer idle time which the tool requires in spite of rapid motion in order to come from one final roll position back into the initial roll position for the next tooth space. This is because, according to the related art, the generating process for each tooth space of a gear is always performed in the same direction for all spiral bevel gears produced through generating. The background for this is the influence of different resultant deflections which the gear cutting machine would be subject to as a function of the cutting process and therefore in the event of alternating rolling directions. On the workpiece, changing resultant deflections lead to indexing errors and to different shape deviations of the flank topography, depending on the direction of roll.
A method for grinding spiral bevel gears in the single-indexing generating method, in which one flank is machined in a downward generating motion to a first reversal point and the other flank of the same tooth space is machined in an upward generating motion to a second reversal point, is known from DE 195 17 360 C1. In this case, downward generating means the roll direction in which the tool moves from the top toward the bottom on a path shaped like a circular arc during the generating process, and upward generating means the corresponding reversed roll direction. However, idle times are not shortened using this generating method, rather, it is important in this case that during the generating process, in spite of the different bevel angles on the grinding wheel, a correct pressure angle and a correct topography arise on both tooth flanks.
In addition, applying the double-roll method during generating of spiral bevel gears is also known. In this case, the cutter head is plunged into the workpiece in a middle roll position in order to remove a large amount of material from the tooth space in a short time, but without reaching the final generating depth. An upward generating motion with chip removal then follows on one tooth flank and then a further infeed in order to reach the final generating depth and the initial roll position. From here, both tooth flanks are now produced through downward generating. This procedure is repeated for each tooth space, so that finally all gaps are manufactured in the same g roll direction, however.
For the generating process for spiral bevel gears, in the Gear Handbook, FIG. 20-2 and FIG. 20-7, machines which operate purely mechanically are described, a generating machine (generator) having a generating drum or cradle and a separate mechanism for tilting the cutter head. In contrast, modern CNC machines for milling or grinding spiral bevel gears, as are described, for example, in DE 196 46 189 C2 or in DE 37 52 009 T2, may achieve this without a generating drum and without a tilt mechanism, solely through spatial motions of the tool carrier and workpiece carrier. For the face-milling method with single-indexing, only five controlled axes are necessary for this purpose, three translational and two rotational. The missing sixth degree of freedom for the general position of a rigid body in space, in this case the cutter head in relation to the workpiece, is the rotation of the cutter head around its rotational axis. It is not necessary as a controlled axis in the face-milling method with single-indexing, because the cutter head is rotationally symmetric and its drive—independently of the other five axes—is only necessary to achieve a desired cutting speed.
Such CNC machines achieve significantly greater operating speeds than purely mechanical bevel gear cutting machines, while simultaneously having more precise setting and travel motions, and are therefore more cost-effective. Nonetheless, the automobile industry requires that the processing times per workpiece be shortened further in order to be able to reduce costs.
Therefore, it is the object of the present invention to implement a method and a machine of the type initially cited in such a way that spiral bevel gears which are produced in the face-milling method with single-indexing may be machined in a shorter time through a generating process than previously, without having to accept noticeable reductions in the precision of the tooth spaces at the same time.